Electrical control circuit for hydropneumatic power mechanisms



Oct. 19, 1954 c. JOHNSON ELECTRICAL CONTROL CIRCUIT FOR HYDROPNEUMATIC POWER MECHANISMS 4 Sheets-Sheet 1 Filed Jan. 9, 1950 Ml VE/V 70R A ffomeys Oct. 19, 1954 c. JOHNS N 2,691,962 ELECTRICAL CONTROL CIRCUIT FOR HYDROPNEUMATIC POWER MECHANISMS Filed Jan. 9, 1950 4 Sheets-Sheet 2 l/VVE/V 78/? Clarence Johnson Oct. 19, 1954 'c. JOHNSON ELECTRICAL CONTROL CIRCUIT FOR HYDROPNEUMATIC POWER MECHANISMS .4 Sheets-Sheet 5 Filed Jan. 9. 1950 R o o m m V MI W m g 0 J v mm 3 N m a, m ww 7 w woT mQ mfi w M v9 Wm? A v mm Wm M m h m ed w m m nw mw 1 f mw Q kw lll J d w; 3 3 mm \EH A Q mm y Al mm AL| & g ml 1 NW5 l. I QM. /M\ vh AR v .2 mm

A Narneys Oct. .19, 1954 2,691,962

C. JOHNSON ELECTRICAL CONTROL CIRCUIT FOR HYDROPNEUMATIC Filed Jan. 9, 1950 POWER MECHANISMS 4 Sheets-Sheet 4 lA/VE/W'OR lg r I06 G/arence Johnsan Aflomeys Patented Oct. 19, 1954 assists OFFICE ELECTRICAL C(PN'JROL CIRCUIT FOR HY- DROPNEUMIATEC PUWER MECHANISMS Application January 9, 1950, Serial No. 137,605

(El. l2l-d5l 8 Claims. 1

The present invention relates to switch mech anisms and control circuits and has particular reference to switch mechanisms and control circuits for hydro-pneumatic power mechanisms.

The novel switch mechanism and control circuits embodied herein are provided for the control of hydro-pneumatic power mechanisms such as those disclosed in my co-pending application, Serial No. 133,922, filed December 19, 1949, cmbodying power feed mechanisms adapted for use in the operation of drill presses and other feeds. In the present embodiment there is provided an improved switch mechanism and control circuit in which recycling of the power feed mechanisms may be controlled manually, automatically in response to dial feed or other indexing mechanisms, or automatically solely in response to the return of the feed mechanism to its normal position. The switch mechanism is actuated in one direction under control of the hydraulic medium and in-the other direction by positive mechanical action in such a manner that recycling can only occur when the working piston of the power mechanism is in its normal or atrest position. This results in a positive and extremely accurate control of the recycling action. Because of the versatility of the switch mechanism and control circuits embodied herein they may be used with equally high efiici ncy in either manual or automatic operation, there is uniformity in the manufacture in the basic switch and circuit structures for either type of operation, and a minimum of production time is lost in changing from one type of operation to the other. It has also been found that because of the positive control incident to the use of the novel switch mechanism and control circuits of this invention there is a substantial reduction in wear and breakage on expensive tools.

With these and other considerations in view it is an important object of this invention to provide a switch mechanism actuated by positive mechanical action on its one side and by oil pressure on its other side.

Another object resides in the provision of a switch mechanism with combined mechanical and hydraulic action for use in connection with hydro-pneumatic power mechanisms.

A further object resides in the provision of a novel combination of switch mechanism and control circuit providing positive cycling action in hydro-pneumatic power feed mechanisms.

A still further object resides in the provision of a switch mechanism and control circuits which result in a material saving in tool life and pro- Euciid, Ohio, assignmduction time in hydro-pneumatic power feed mechanisms.

Another object resides in the provision of a switch mechanism and control circuit for use in hydro-pneumatic power feed mechanisms that may be manually operated or fully automatic.

A further object resides in the provision of a switch mechanism for use in hydro-pneumatic power feed mechanisms that is of low cost, easily fabricated and eflicient in operation.

Other objects and advantages will be apparent from the following description in conjunction with the accompanying drawings and from the appended claims.

The accompanying drawings, in which like reference numerals are used to designate similar parts throughout, illustrate a preferred embodiment for the purpose of disclosing the invention. The drawings, however, are not to be taken in a limiting or restrictive sense, it being apparent to those skilled in the art that various changes in the illustrated construction may be resorted to without in any way exceeding the scope of the invention.

In the drawings:

Fig. 1 is a schematic diagram of a hydro-pneumatic power feed system showing the switch mechanism and control circuit; Fig. 1 illustrates the system at the end of a retract stroke;

Figs. 2 and 3 are schematic diagrams similar to Fig. 1 illustrating the end of rapid approach and start of controlled feed rate, and start of a retract stroke, respectively;

Fig. l is a top view of a switch mechanism with parts broken away to show its operation;

Fig. 5 is a side elevation of a switch mechanism;

Fig. 6 is a schematic diagram illustrating a modified starting button arrangement for automatic recycling; and

Fig. 7 is a schematic diagram illustrating another modified arrangement for automatic recycling.

Having reference to Figs. 1, 2 and 3 of the drawings, a detailed discussion of the operation of the hydro-pneumatic power feed mechanism shown may be had from my co-pending application, S. N. 133,922. However, in order to understand fully the function and operation of the switch a brief description of this power feed mechanism follows.

Fig. 1 illustrates the power feed system with working piston l t in its starting or fully retracted position. As viewed in Figs. 1, 2 and 3 piston is is acted upon by compressed air on its left side and by oil on its right side. Arrows showing the direction of flow of the air are indicated at 12, while arrows showing the direction of flow of the oil are indicated at 14. The direction of fiow of the air is controlled by the 4-way solenoid operated air valve generally indicated at 15. The arrows l2 and I4 of Fig. 1 indicate that on completion of a retract stroke following a power stroke the oil acting on the right side of piston 19 has pushed the piston to its fully retracted starting position and that the cycle will be repeated as soon as starting button l6, normally open, is manually or automatically operated. As soon as start button 16 is operated piston valve IT in air valve 15 will move to the position shown in Fig. 2, by means to be more fully explained hereinafter, and a new cycle will be started.

Referring now to Fig. 2 it will be seen that compressed air entering air valve l5 through the air supply intake l8 will be directed by piston valve 11 through passage Hi to the left side of piston ID forcing it to the right in a power stroke. Movement of piston 16 to the right forces oil in the right side of the working cylinder 29 to escape through passages 22 and 24. The major portion of the oil passes through passage 22 entering rapid approach cylinder 25 where it acts on piston 26 forcing it to the left which causes air in the left side of cylinder 25 to exhaust through passages 21, 28 and air valve exhaust vents 29.

As is fully explained in my co-pending application, S. N. 133,922, the rapid approach cylinder is provided to allow for rapid approach of the quill or tool 30 to the work piece. Mounted in cylinder 25 is an adjustable stop 32 adjusted to arrest the leftward movement of piston 25 just as the tool 30 starts to contact the work piece. At this point, the oil being unable to force piston 26 farther to the left must pass through an adjustable metering orifice 34 into the feed cylinder 35 forcing piston 35 to the left. Movement of piston 35 to the left causes air in the left side of cylinder 35 to exhaust through passages 31, 28, and air valve exhaust ports 23. Also connecting passage 22 with cylinder 35 is a passage 38 provided with a one-way check valve 39 so that oil from passage 22 cannot enter cylinder 35 on a power stroke of piston H] but must enter through metering orifice 34 alone. This means that after the initial rapid approach of the tool to the work piece further advance of the tool into the workpiece is controlled and slower, the desired speed being determined by the adjustment of metering orifice 34. Fig. 2 illustrates the positions of pistons I0, 26 and =36 at the end of the rapid approach and start of the controlled feed rate.

As piston I0 is pushed by the air to the right during a power stroke or at any other time when air pressure is applied to either the power piston or the control pistons 26 and 36, a portion of the oil in the right side of cylinder 29 will be forced through passage 24 into the switch mechanism generally indicated at 40. Piston l6 continues to the right until quill 36 strikes the adjustable stop 4| as shown in Fig. 3, which illustrates the system immediately after the initiation of a retract stroke. After striking stop 4| the quill dwells or takes approximately 12 revolutions while the oil pressure drops due to the continued bleed of oil through passage 34 and causes the switch in switch mechanism 40 to be tripped in a manner to be explained fully hereinafter. The tripping of the switch in turn causes piston valve IT in air valve is to assume the position shown in Figs. 1 and 3.

Referring to Fig. 3 it will be observed that the air will now be directed from air supply intake [8 through passages 28, 21 and 31 into the left sides of cylinders 25 and 35 causing movement of pistons 25 and 36 to the right. Movement of piston 26 to the right forces oil in the right side of cylinder 25 through passage 22 and into the right side of working cylinder 20. Movement of piston 36 to the right forces most of the oil in the right side of the cylinder 35 through one-way check valve 39, passages 38 and 2 into the right side of cylinder 20. A small amount of oil will also be forced through metered orifice 34 but since most of the oil passes through passage '38 the desired rapid retraction of the piston l6 and quill or tool 30 is obtained as is explained in detail in my co-pending application Serial No. 133,922.

The oil pressure on the right side of piston i0 causes the piston to move to the left retracting the tool 36 and forcing air in the left side of cylinder 20 to exhaust through passage [9 and air valve exhaust ports 42. At the end of the retracting stroke piston Ill is in its fully retracted or starting position shown in Fig. 1 and the power feed mechanism is ready to be recycled.

Mounted within switch mechanism 40, Figs. 1-5, is a snap switch 44 of any suitable type such as those shown in U. S. Patent No. 2,260,964 to Wilms and U. S. Patent No. 2,304,400 to Eaton. Snap switch 44 has fixed contacts 45 and 46 and a movable contact 41 secured to a spring toggle arm 48 which is fastened to one side of switch 44 at 49. Contact 46 is connected by means of the screw 56, Fig. 4, to lead wire 52 of the control circuit and contact 41 is connected through arm 48 and screw 54 to lead wire 55 of the control circuit. Contact 45 is shown as a stop only, although it may be used to perform some operating function in the system if desired. When the contacts are in the position shown in Figs. 1, 2 and 4 the circuit through the switch is closed, and when the contacts are in the position shown in Fig. 3 the circuit through the switch is open.

A reset button 58 is mounted with a loose fit in a bore at one side of the switch mechanism 40. Reset button 58 is held in position by a spring 59 formed at one end with a yoke which fits into an annular groove 66 in the reset button 58. Spring 59 is secured at its other end to the side wall of switch mechanism 40 at 62. The spring 55 tends to force the button 58 outwardly of the switch so that in its normal position the top of the button will be in the path of the rearward extension 64 of piston 10 as shown in Figs. 2 and 3. When piston H1 is in its starting or fully retracted position the extension 64 pushes reset button 58 into the switch against the action of its spring 59 as shown in Figs. 1 and 4. When button 58 is thus forced inwardly it pushes on switch button 65 mounted loosely within a bore in switch 44 causing contacts 46 and 41 to close as shown in Figs. 1 and 4. As soon as the switch is closed the power feed mechanism is ready to recycle whenever the start button [6 is closed manually or automatically, and when the recycling starts, piston ID and extension 64 will be advanced during the power stroke, as explained hereinbefore, allowing button 58 to be again raised by the action of spring 59 into the path of extension 64. It should be noted here, however, that even though button 58 has been returned by its spring to its normal outward posiclosed as is best illustrated in Fig. 2. The switch will remain closed until it is positively pushed into its open position.

When air pressure is applied either to piston I or control pistons 29 and 39 oil is forced through passage 24 into switch mechanism 40 as explained hereinbefore. The oil enters switch mechanism 40 through a tapped hole 66, as is best shown in Fig. 4, into which the passage or conduit 24 is threaded. At the bottom of hole 96 there is a small passage 61 leading into a bore or cylinder 98 in the side of switch mechanism 40. A spring biased piston assembly 69 and a gland 70 are fitted into the cylinder 68 as shown in Fig. 4. At the outer end of piston 59 there is an adjustable piston stop screw II which is locked in position by means of lock nut 12 when the proper adjustment is obtained. The oil entering cylinder 98 through passage 61 passes into an annular groove 14 in the cylinder 58 formed by gland 70 where, as the oil pressure is built up at the beginning of the power stroke, it will press against the shoulder 15 of the piston forcing it outwardly of switch mechanism 40. Piston 69 is normally urged inwardly by means of piston tension spring 16 which is provided at one end with a yoke which fits into anannular groove 71 in piston 69. The other end of spring 16 is secured by some means such as a cap screw I8 to cylindrical piece 19 slidably fitted into a bore 80 in the side wall of switch mechanism 40. At its outer end, piece I9 abuts the adjustable spring tension screw 82 which is locked in position by means of lock nut 84. The position of screw 82 is important since'it is this adjustment which controls the point at which oil pressure acting on the piston must drop before the switch is reversed. This reverses the air valve I as will be more fully explained hereinafter, and causes working piston I0 to be returned to the starting position. If the tension in the piston spring I6 is not great enough the quill or tool 30 may dwell too long or stop completely at the end of the power stroke. If the tension in piston spring I9 is too great, the unit will oscillate rapidly without completing a full power stroke.

In the absence of pressure in the hydraulic circuit, piston assembly 69 is held inwardly by its tension spring I9 as shown in Fig. 1. However, when the oil is forced into the switch mechanism 40, for example, during the power stroke pressure will be built up which forces piston 99 outwardly against the action of its tension spring I9, as shown in Fig. 2, until it strikes against its stop 1 I. In actual practice the movement of piston 69 is very small as maybe seen from the small amount of clearance between the top of the piston and the gland in Fig. 4. When the oil pressure starts to drop at the end of the power stroke, as explained hereinbefore, tension spring 16 overcomes the pressure of the oil and causes piston 59 to surge inwardly and strike switch button 85; button 85 in turn strikes spring arm 48 causing it to reverse its position which opens contacts 46 and 41, Fig. 3, so that the circuit through switch 44 is open. It is to be understood that since this takes place at the end of the power stroke when extension 64 is not holding reset button 58, the button 58 and switch button 65 will offer no resistance to the force exerted by button 85 against spring arm 48.

The gland I0 mounted'in the upper part of cylinder 68 is pressed into the bore of cylinder 68 and is necessary in order to properly prepare the bore for the piston since the bore is shallow,

and in order to have accurate piston action the diameter must be very true and highly polished to avoid excessive wear on the O-rings 86. The O-rings 86 are conventional and prevent oil leakage along the sides of cylinder 68.

With reference to Figs. 1, 2 and 3, current enters the control circuit through power lines 90 and 92 which lead to the air valve solenoid coil 94. The lead wire 55 connects line 90 and movable contact 41 in the switch mechanism 40, see also Fig.4, and has in series with it relay coil 95. Lead 96 connects line 92 with one side of start button I8; the other side of start button I6 is connected by lead wire '52 to fixed contact 46 in switch mechanism 40 so that when the start button I0 is depressed and the switch 44 in switch mechanism 40 is closed, Figs. 1, 2 and 4, a circuit will be completed through line 90, lead 55, switch 44, lead 52, start button I0, lead 96, and line 92. When this circuit is completed relay coil 95 will be energized and its armature 9'! will close contacts 98 in line 90, contacts 99 in line 92 and contacts I00 in a holding circuit. The holding circuit comprises lead wire I02, connected to lead 96 and lead wire I04 connected to lead 52, and is provided so that after start button I6 is released a circuit will be maintained through line 90, lead 55, relay 95, lead 55, switch 44, lead 52, lead I04, contacts I00, lead I02, lead 96, and line 92. This circuit will remain closed until switch 44 is opened.

When the contacts 98 and 99 in lines 90 and 92, respectively, are closed by relay armature 9'1 the air valve solenoid coil '94 is energized and piston valve I1 is pulled to the left against the action of its spring I05 and takes the position shown in Fig. 2.

Referring now to Fig. 1 wherein the system is shown with piston I0 in its fully retracted starting position it will be seen that switch 44 is closed since rearward extension 64 has depressed reset button 58, as explained hereinbefore. With piston It in this position the cycle is started by pushing start button I9 which completes the circuit through relay 95 which in turn causes solenoid coil 94 to be energized, and coil 94 pulls piston valve IT to the left against the action of its spring I05. Piston valve I'i is then in the Fig. 2 position causing the air to push piston I0 to the right in a power stroke as explained above, forcing oil into the switch mechanism 49, and at the end of the stroke the oil pressure drops surficiently to allow piston 69 to spring up and open switch 44. The exact point at the end of the power stroke where this will take place is dependent upon the adjustment of piston tension spring I6 as explained above. When the circuit through switch 44 is opened relay 95 is deenergized and this in turn deenergizes solenoid coil 94 allowing piston valve I! to be moved to the right by the action of its spring I05. Piston valve I! will then be in the position shown in Fig. 3 and the oil acting on the right side of piston I0 will cause it to retract to its Fig. 1 or starting position. When piston I0 has completed its travel to the left depression of reset button 58 by extension 64 will again close switch 44 allowing a new cycle to be started whenever start button I5 is again depressed. Thus it will be understood that recycling cannot take place until the piston I0 is fully retracted to its starting position where extension 64 can act on reset button 59 to close switch 44.

As shown in Fig. 6 start button I6 may be operated automatically as well as manually. 'In the embodiment shown button i6 is provided with a roller lDB which follows a cam wheel such as I01. A spring Hi8 tends to hold button IS in its open position and hold the roller I06 against the periphery of cam wheel I (ll. Appropriately spaced rises 109 on cam wheel 10'! cause start'button to periodically open and close as wheel H11 rotates in the direction shown by the arrow. Cam wheel I01 may be mounted in any suitable manner on whatever assembly may require automatic recycling. For instance, if the hydropneumatic mechanism is to be used on a hopper fed, dial feed arrangement the cam wheel Ill! can be mounted below and geared to the dial so that the rises H19 on wheel ill! will cause the button Hi to close each time the dial carries a Work piece to an operating station.

A further modification is shown in Fig. '7 which illustrates a portion of the control arranged so that the power feed mechanism may be controlled automatically solely in response to the return of working piston to its fully retracted or starting position. In this arrangement the start button I6, contacts I of the holding circuit, and lead wires 96, I02 and H14 have been eliminated from the circuit. Lead wire 52 which is connected at its one end to contact 46 in snap switch 44, as shown in Fig. 1, is now connected directly with power line 92 at its other end as shown in Fig. '7. In other respects the modified arrangement is like the arrangement shown in Fig. l with lead 55 connected to movable contact 4? in switch 44, and lines 90 and 92 carrying power from the power supply to the solenoid coil 94.

With piston If! in its fully retracted starting position, Fig. l, the switch 44 is closed, as explained hereinbefore, and current will pass through line 99, lead 55, relay 95, lead 55, switch 44, lead 52 and line 92 to complete the circuit and energize relay 95. Energizing relay 95 causes contacts 98 and 99 to be closed so that current from the power supply energizes solenoid coil 94 causing valve I! to shift so that piston is started on a power stroke. At the end of a power stroke piston 59 causes switch M to be opened, as explained hereinbefore, thereby deenergizing relay 95 and solenoid coil 94 so that valve I! is again shifted causing piston in to return to its starting position where it will close switch 34 starting a new cycle. To start and stop the operation of the power feed in this arrangement a line switch H38 of any suitable design is provided across lines Si; and 92 from the power supply.

The switch mechanism is an important link in automatic operation when a number of drill units, which is usually the case, are used on one machine. The units a-re usually elementsof a machine rather than a machine in themselves, and the controls are one of the features that make it easy to integrate them into the one machine. One solenoid valve will usually serve all the units and, in that case, the switches are connected in parallel and the solenoid valve will not reverse until the last unit has completed its work. If this function fails some holes may not be drilled deep enough before the units retract. On the return stroke the units must sometimes be interlocked with the indexing mechanism, clamps, fixtures or work feeders to prevent them from operating until the units are fully retracted. If the index table starts before the drills are retracted 8 .they will be broken. The last unit to completely retract starts the index table and when the index table is. completely in position it starts all the units again. This cycle will continue until stopped by hand or breakdown.

It will be understood from the foregoing description that there is provided a novel switch mechanism and control circuit for use in hydropneumatic mechanisms which switch is actuated in one direction by positive mechanical action and in the other direction under control of oil pressure. The switch mechanism and control circuit may be manually or automatically recycled allowing them to be used in a variety of installations, and provide positive and accurate recycling action for the power feed mechanisms. In addition, the provision of a positive mechanical resetting action results in increased production and longer tool life over the switch mechanisms of the prior art.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Th present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A power mechanism comprising a power member movable through an advance and retract stroke under the control of a closed hydraulic circuit, the pressure in said circuit dropping when said advance stroke is interrupted, a valve selectively operable to supply fluid to said mechanism to advance and retract said power member, an electrical control circuit including a solenoid for actuating said valve, a starting switch in said circuit adapted, when closed, to energize said control circuit and said solenoid to shift said valve and supply fluid to advance said power member, switch mechanism in said control circuit, means responsive to a predetermined drop in the pressure in said hydraulic control circuit for actuating said switch mechanism to deenergize said electrical control circuit whereby said valve is shifted to supply fluid to retract said power member, and additional mechanism responsive to the movement of said power member 'to its retracted position to reset said switch mechanism to its initial position whereby another cycle may be initiated by closing said start switch.

2. The control circuit according to claim 1 together with means for automatically periodically closing saidstarting switch whereby a number of complete cycles may be periodically automatically initiated.

3. A power mechanism comprising a power member movable through an advance and retract stroke under the control of a closed hydraulic circuit, the pressure in said circuit dropping when said advance stroke is interrupted, a valve selectively operable to supply fluid to said mechanism to advance and retract said power memher, an electrical control circuit including a solenoid for actuating said valve, a normally open start switch adapted to be selectively closed to energize the control circuit and said solenoid to shift said valve and initiate the advance stroke of said power member; a holding circuit to maintain the control circuit energized after said start switch has been released; a doubl contact switch mechanism; means responsive to a predetermined pressure drop in said hydraulic circuit to actuate said double contact switch mechanism and open said holding circuit to deenergize the control circuit and said solenoid and thereby shift said valve to cause the power member to return to its starting position; and additional means to reset the control circuit whereby the power member may be recycled when the start element is again engaged.

4. A power mechanism comprising a power member movable through an advance and a retract stroke under the control of a closed hy draulic circuit, the pressure in said circuit dropping when said advance stroke i interrupted, a valve selectively operable to supply fluid to said mechanism to advance and retract said power member, a control circuit including a solenoid for actuating said valve, a normally open start switch adapted to be selectively engaged to energize the control circuit and said solenoid to shift said valve to advance said power member; a holding circuit to maintain the control circuit energized after said start element has been released; a switch mechanism in said control circuit; hydraulic means responsive to a predetermined drop in pressure in said hydraulic circuit operable to actuate said switch mechanism to open said holding circuit and thereby open said control circuit whereby said valve is shifted to cause the power member to retract, and means operable when said power member is fully retracted to close said switch mechanism thereby placing said control circuit in it initial condition whereby the power member may be recycled when the start switch is again engaged.

5. In a hydropneumatic power system, a power piston movable through an advance and retract stroke under the control of fluid flow in a closed hydraulic circuit, the pressure in said closed circuit dropping at the end of an advance stroke of said power piston, feed control pistons in said closed circuit, valve means for selectively applying fluid pressure to said power piston or said control pistons, respectively, to advance or retract said power piston: an electrical control circuit including a solenoid for operating said valve means, a starting element adapted when closed to energize said control circuit and said solenoid to operate said valve to apply pressure to advance said power piston, a double contact switch in said circuit, a hydraulic actuator for said switch connected to said hydraulic circuit and adapted to be operated by a preselected pressure drop therein to actuate said switch to open one contact and deenergize said circuit whereby said solenoid operated valve will be deenergized and apply pneumatic pressure to said feed control pistons to retract said power piston, and mechanical means for actuating said switch mechanism at the completion of the retract stroke of said power piston to close said one contact thereby restoring the electrical control circuit to its initial condition whereby the power piston may be recycled by closing said start element.

6. An electrical control circuit including a solenoid valve operator comprising, a normally open starting switch adapted to be selectively closed to energize the control circuit and said solenoid valve operator; a holding circuit to maintain the control circuit energized after said starting switch is opened; a dual switch mecha nism in said holding circuit; pressure responsive means operable to actuate said dual switch mechanism upon a drop in pressure below a predetermined minimum to open holding circuit to deenergize said solenoid valve operator; and mechanical means to close said dual switch mechanism after deenergization of said solenoid valve operator and close the holding circuit thereby placing said control circuit in its initial condition for a repeat operation.

7. A power mechanism comprising a power member movable through an advance and retract stroke under the control of th resistance flow of a fluid in a closed hydraulic circuit, the pressure in said hydraulic circuit dropping when said advance stroke is interrupted; a valve selectively operable to supply fluid to said mechanism to advance and retract said power member; an electrical control circuit, including a solenoid for actuating said valve, adapted to be connected to a source of electrical energy and, when energized, to energize said solenoid to shift said valve and supply fluid to advance said power member; a switch in said electrical control circuit; means responsive to a predetermined drop in th pressure in said hydraulic circuit for actuating said switch in one direction to deenergize said solenoid whereby said valve is shifted to supply fluid to retract said power member; and additional means responsive to the movement of said power member to its retracted position to actuate said switch in the opposite direction to reset said switch to its initial position.

8. A power mechanism comprising a power member movable through an advance and retract stroke underthe control of the resistance flow of a fluid in a closed hydraulic circuit, the pressure in said hydraulic circuit dropping when said advance stroke is interrupted; a valve selectively operable to supply fluid to said mechanism to advance and retract said power member; an electrical control circuit, including a solenoid for actuating said valve, adapted to be connected to a source of electrical energy and, when energized, to energize said solenoid to shift said valve to advance said power member; a holding circuit to maintain the solenoid energized during the advance stroke of said power member; a switch in said electrical control circuit; hydraulic means responsive to a predetermined drop in pressure in said hydraulic circuit operable to actuate said switch in one direction to open said holding circuit and thereby deenergize said solenoid whereby said valve is shifted to retract said power member; and means operable when said power member is fully retracted to actuate said switch in the opposite direction to reset said switch to its initial position.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,724,635 Bath Aug. 13, 1929 1,743,782 Kershaw Jan, 14, 1930 1,927,583 Ernst Sept. 19, 1933 2,169,470 Miller et a1 Aug. 15, 1939 2,219,896 Harrington et al. Oct. 29, 1940 2,252,939 McCoy Aug. 19, 1941 2,418,508 Goepfrich Apr. 8, 1947 2,448,557 Stephens Sept. 7, 1948 2,507,065 Trautman May 9, 1950 2,574,096 Fischer et al. Nov. 6, 1951 

